A conventional manual transmission for a motor vehicle includes a clutch and a gearbox. The clutch functions to temporarily disengage delivery of torque from an engine crankshaft to the gearbox, in order that the gears may be shifted. Typically, the gearbox comprises an output shaft connected to the drive wheels of the vehicle, a layshaft or countershaft, and a clutchshaft or input shaft which is rotated by the engine whenever the clutch is engaged. Both the clutchshaft and the output shaft are provided with gears in constant mesh with mating gears on the countershaft, and the output shaft turns at some ratio of the engine crankshaft depending on which gears are established as the driving connection between the countershaft and the output shaft.
The noise created by the rotation of unloaded meshing gears, known as idle rollover, is a problem with conventional manual transmissions. Regardless of the state of the clutch or the position of the gear selector, the gears on the countershaft remain in mesh with the gears on the clutchshaft and output shaft at all times. Whenever the gear selector is in a neutral position and the clutch is engaged, noise results because these gears are rotated by the engine crankshaft. This noise is compounded by engine idle speed changes which occur continually without any action by the operator.
Various prior art patents teach designs in which the transmission may not be put into gear while the clutch is engaged. For instance, U.S. Pat. No. 3,265,170 discloses a vehicle clutch-to-transmission shifting assembly interlock wherein the transmission cannot be shifted until the clutch pedal is depressed. Similarly, U.S. Pat. No. 3,522,867 discloses a fluid pressure operated clutch and gears with interlock in which the transmission cannot be put into gear or taken out of gear before the clutch is disengaged. Other designs teach devices for holding the clutch disengaged when shifting gears. U.S. Pat. No. 3,011,357 teaches a clutch-controlled automatic gear shifting mechanism which is hydraulically operated and in which reengagement of the clutch is not possible until after the gear shifting operation has been completed.